Description The three-spined stickleback, Gasterosteus aculeatus, is a fish native to much of northern Europe, northern Asia and North America. It has been introduced into parts of southern and central Europe.

Three-spined sticklebacks have recently become a major research organism for evolutionary biologists trying to understand the genetic changes involved in adapting to new environments. The entire genome of a female fish from Bear Paw Lake in Alaska was recently sequenced by the Broad Institute and many other genetic resources are available. This population is under risk from the presence of introduced northern pike in a nearby lake.

Dimensions Up to 4" (10 cm).

Endangered Status The Unarmored Threespine Stickleback, a subspecies of the Threespine Stickleback, is on the U.S. Endangered Species List. It is classified as endangered in California. The primary causes of this species' decline are the introduction of exotic fishes, which prey on it and compete with it for food; destruction or degradation of its habitat by urban development and water-management practices; and its intergrading with other subspecies of sticklebacks.